The object of the present invention is to provide a process for pretreating or purifying a gas stream consisting of atmospheric air prior to the cryogenic separation of the air, particularly by cryogenic distillation.
It is known that atmospheric air contains compounds that have to be removed before the air is introduced into the heat exchangers of the cold box of an air separation unit, especially the compounds carbon dioxide (CO2), water vapour (H2O) and/or hydrocarbons (CnHm) for example.
This is because, in the absence of such an air pretreatment for removing its CO2 and water vapour impurities, these impurities condense and solidify as ice when the air is cooled to cryogenic temperature, which may result in problems of the equipment, especially the heat exchangers, distillation columns, etc., becoming blocked.
Furthermore, it is also common practice to remove the hydrocarbon impurities liable to be present in the air so as to avoid any risk of deterioration of the equipment, particularly of the distillation column or columns located downstream of the cold box.
At the present time, this air pretreatment is carried out, depending on the case, by a TSA (Temperature Swing Adsorption) process or by a PSA (Pressure Swing Adsorption) process; the expression xe2x80x9cPSA processxe2x80x9d should be understood to mean actual PSA processes, VSA (Vacuum Swing Adsorption) processes, VPSA (Vacuum Pressure Swing Adsorption) processes and similar processes.
Conventionally, a TSA process cycle for purifying air comprises the following steps:
a) purification of the air by adsorption of the impurities at a superatmospheric pressure and at ambient temperature;
b) depressurization of the adsorber down to atmospheric pressure or below atmospheric pressure;
c) regeneration of the adsorbent at atmospheric pressure, especially by residual gases or waste gases, typically impure nitrogen coming from an air separation unit and heated to a temperature above +80xc2x0 C. by means of one or more heat exchangers;
d) cooling of the adsorbent to ambient or subambient temperature, especially by continuing to introduce therein to the waste gas coming from the air separation unit, but the gas not being heated;
e) repressurization of the adsorber with purified air coming, for example, from another adsorber which is in production phase.
As regards a PSA process cycle for purifying air, this usually comprises substantially the same steps a), b) and e), but differs from a TSA process by the absence of a step for heating the waste gas or gases during the regeneration step (step c)), and therefore the absence of step d), and, in general, a shorter cycle time than in the TSA process.
In general, air pretreatment devices comprise two adsorbers, operating alternately, that is to say one of the adsorbers is in production phase while the other is in regeneration phase.
Such TSA air purification processes are especially described in U.S. Pat. No. 3,738,084 and FR-A-77/25845.
In general, the removal of the CO2 and the water vapour is carried out over one or more beds of adsorbents, preferably several beds of adsorbents, namely generally a first adsorbent designed to preferentially stop the water, for example a bed of activated alumina, of silica gel or of zeolites, and a second bed of adsorbent for preferentially stopping the CO2, for example a zeolite. In this regard, mention may especially be made of documents U.S. Pat. No. 5,531,808, U.S. Pat. No. 5,587,003 and U.S. Pat. No. 4,233,038.
However, it is not an easy matter to achieve effective removal of the CO2 and water vapour which are contained in the air over one and the same bed of adsorbent since water has an affinity for the adsorbents which is markedly greater than that of CO2, and it is therefore customary to use at least two beds or layers of adsorbents of different types.
Thus, it is common practice to use a zeolite of the 13X type for removing the CO2 since the 13X zeolite is reputed to be particularly effective for stopping small amounts of CO2 and possibly of water, as it has a strong affinity and selectivity for these polar molecules. In addition, the X zeolite has among the largest micropore diameters, enabling it to adsorb, with favourable kinetics, molecules having a kinetic diameter up to 0.8 nm, as mentioned by D.W. Breck""s document xe2x80x9cZeolite molecular sievesxe2x80x9d, Krieger Publishing Company, 1984, p. 612.
However, the 13X zeolite is unable to stop all undesirable molecules liable to be present in a gas stream.
This is because the gas molecules adsorbed by the 13X zeolite are essentially, and in increasing affinity: methane, ethane, propane, nitrogen protoxide, ethylene, carbon dioxide, butane, propylene (C3H6), acetylene (C2H2), toluene and methylcyclohexane.
In this regard, reference may be made to the following documents: E. Alpay, xe2x80x9cAdsorption parameters for strongly adsorbed hydrocarbon vapours on some commercial adsorbentsxe2x80x9d, Gas Sep. and Purif., Vol. 10, No. 1, pp 25 (1996); G. Calleja, xe2x80x9cMulticomponent adsorption equilibrium of ethylene, propane, propylene and CO2 on 13X zeolitexe2x80x9d, Gas Sep. and Purif., Vol. 8, No. 4, p. 247 (1994); V. R. Choudhary, xe2x80x9cSorption isotherms of methane, ethane, ethene and carbon dioxide on NaX, NaY and Na-mordenite Zeolitesxe2x80x9d, J. Chem. Soc. Faraday Trans., 91(17), p. 2935 (1995); and A. Cointot, P. Cartaud, C. Clavaud, xe2x80x9cEtude de l""adsorption du protoxyde d""azote par diffxc3xa9rents tamis molxc3xa9culaires [Study of the adsorption of nitrogen protoxide by various molecular sieves]xe2x80x9d, Journal de Chimie Physique, Vol. 71, No. 5, p. 765-770 (1974).
It therefore follows that an industrial air-prepurification unit strictly dimensioned for stopping carbon dioxide with a standard zeolite, typically a 13X or 5A zeolite, only partially stops ethylene, propane and nitrogen protoxide, as indicated by Dr J. Reyhing""s document xe2x80x9cRemoving hydrocarbons from the process air of air separation plants using molecular-sieve adsorbersxe2x80x9d, Linde Reports on Science and Technology, 36/1983.
Similarly, this situation for hydrocarbons is also described by Dr J. Reyhing in the above document.
Likewise, with regard to nitrogen protoxide, the ineffectiveness of the 5A zeolite for stopping N2O compared with CO2 has been demonstrated by U. Wenning in xe2x80x9cNitrous oxide in air separation plantsxe2x80x9d, MUST""96, Munich Meeting on Air Separation Technology, Oct. 10-11, 1996.
Moreover, there are also similar problems with ethylene, which is an unsaturated hydrocarbon unstable in the presence of oxygen, soluble in liquid oxygen to a level of 30,000 ppm with a low solute-gas equilibrium coefficient, its freezing point being xe2x88x92169xc2x0 C., whereas that of liquid oxygen at 1.2 bar is xe2x88x92181xc2x0 C.
It may therefore be readily understood that, if the prepurification plant does not completely stop the ethylene, it may easily be found downstream of this plant and damage to the cryogenic distillation plant, particularly the distillation columns and/or reboilers, may then result therefrom, something which is unacceptable.
Similar problems may also arise with ethane and propane which may be found in the liquid state at the liquid oxygen temperature at a pressure of 1.2 bar.
Although certain documents provide more or less effective solutions allowing some of the impurities that may be found in a stream of atmospheric air to be removed, the problem of effective removal, that is to say complete stoppage of propane, ethylene and/or nitrogen protoxide which are contained in a gas stream, particularly an air stream, has not yet been solved.
Indeed, document EP-A-847,792 provides a process for adsorbing acetylene impurities and C3-C8 hydrocarbons, in a PSA cycle, no matter whether a CaX, CaA, 5A, 13X or Na-mordenite zeolite is used. However, it should be emphasized that this type of adsorbent gives, overall, results that are less satisfactory than an activated-alumina/NaY double bed.
Moreover, documents EP-A-766,991 and EP-A-453,202 propose the use of standard activated alumina or activated alumina which is thoroughly treated for better air purification by a PSA process. However, the impurities mentioned are only water and CO2.
Furthermore, documents U.S. Pat. No. 5,587,003 and U.S. Pat. No. 5,531,808 describe the use of clinoptilolite or LSX zeolites for stopping carbon dioxide in a stream of atmospheric air and at an ambient temperature up to 80xc2x0 C. The choice of cations used is extremely wide, namely Groups I, II and IIA, as well as Group IIIB, but no indication is given as regards the possible influence of a particular choice of ions for stopping impurities other than CO2.
Similarly, several documents describe stopping ethylene or nitrogen protoxide by zeolitic adsorbents.
Thus, document DD-A-6225345 describes the use of activated charcoal and of a CaA zeolite for purifying the atmosphere of chambers for preserving fruits and vegetables. This is because the decomposition of plant matter releases ethylene which has to be extracted in order to extend its period of preservation.
Other articles describe stopping nitrogen protoxide by the 5A zeolite.
Thus, mention may be made of the document by F. Mayinger and R. Eggert-Steger xe2x80x9cExperimentelle und theorische Untersuchungen zur Koadsorption von CO2 und N2O am Molekularsieb 5A [Experimental and theoretical investigations on coadsorption of CO2 and N2O on 5A molecular sieve]xe2x80x9d, Vol. 27, No. 7, pp. 405-412, 1992, in which it is specified that the N2O adsorptivity of the 5A zeolite is less than its CO2 adsorptivity.
Document SU-A-1,357,053 teaches the use of Ba-mordenite rather than 5A zeolite for removing nitrogen protoxide, the regeneration being carried out above 180xc2x0 C.
Mention may also be made of the document, by S.D. Kovalev, S.E. Starobinets, R.G. Kefer and I.S. Puzanov, xe2x80x9cCaractxc3xa9ristiques statiques et dynamiques de l""adsorption du protoxyde et de l""oxyde d""azote sur des sorbants rxc3xa9sistants aux acides [Static and dynamic characteristics of protoxide adsorption and nitrogen oxide on acid-resistant sorbents]xe2x80x9d, Vol. 1, pp. 38-41, 1982, in which it is shown that H-mordenite has a useful N2O adsorptivity but has, on the other hand, a very low adsorptivity for carbon dioxide.
The document, by O. M. Dzhigit, A. V. Kieselev, T. A. Rachmanova and S. P. Zhanov, xe2x80x9cInfluence of Li+, Naxe2x88x92and K+ Cation Concentrations in X and Y Zeolites on Isotherms and Heat of Adsorption of Propane and Waterxe2x80x9d, Journal of the Chemical Society, Faraday Transactions, 1979, describes the adsorption of propane on X and Y zeolite as a function of the cation used: the cations are Li+, Na+ and Kxe2x88x92.
Furthermore, document EP-A-294,588 teaches the use of a zeolite with strontium cations for stopping the CO2 in air.
Stopping the CO2 of a gas stream on BaX is described in document U.S. Pat. No. 3,885,927.
However, these two publications do not mention stopping N2O, C3H8 or C2H4.
To summarize, it seems that the problem of removing propane, ethylene and/or propylene impurities liable to be present in a gas stream, particularly a stream of air, has not been solved satisfactorily up until now.
The object of the present invention is therefore to solve the above problem, that is to say to provide an adsorption process for purifying a gas stream containing propane, ethylene and/or nitrogen protoxide impurities and, possibly, carbon dioxide and/or water vapour impurities liable to be present in this gas stream.
In other words, the object of the invention is not to propose an adsorbent suitable for optimally stopping N2O, C2H4 and/or C3H8, but to propose an adsorbent which does stop any CO2 possibly present as well as a 13X (NaX) zeolite does and to stop completely, upon CO2 breakthrough, the C3H8, C2H4 and N2O type impurities, contrary to what may be obtained by means of a 13X zeolite which is not the most suitable zeolite for completely stopping, simultaneously, the carbon dioxide, propane, ethylene and/or nitrogen protoxide in a gas stream, such as a stream of atmospheric air.
The invention therefore relates to a process for purifying a gas stream containing at least one impurity chosen from the group formed by propane, nitrogen protoxide and ethylene, in which process:
(a) the gas stream to be purified is brought into contact with at least one adsorbent comprising at least one X zeolite containing barium cations;
(b) at least one impurity is adsorbed on the adsorbent.
Depending on the case, the process of the invention may comprise one or more of the following characteristics:
the X zeolite contains at least 30% of barium cations, preferably at least 70% of barium cations and even more preferably from 75 to 98% of barium cations;
the X zeolite has a pore size ranging between 4 and 10 xc3x85, preferably between 5 and 8 xc3x85;
the zeolite furthermore contains at least one cation of Group IA or IIA, preferably potassium, lithium, calcium, strontium, magnesium and/or sodium cations;
the zeolite has an Si/Al ratio ranging between 1 and 1.40, preferably ranging between 1 and 1.15, preferably about 1;
the stream of gas is an air stream;
the gas stream to be purified furthermore contains at least one impurity chosen from carbon dioxide and water vapour;
at least one impurity chosen from carbon dioxide and water vapour is furthermore adsorbed and preferably carbon dioxide is adsorbed on the X zeolite containing barium cations;
the gas stream is at a temperature ranging between xe2x88x9240xc2x0 C. and +80xc2x0 C.;
the adsorption pressure ranges between about 3 bar and 40 bar, preferably between 5 bar and 25 bar;
the desorption pressure ranges between about 0.5 bar and 6 bar, preferably between 1 bar and 1.4 bar;
the flow rate of the gas stream ranges between 1 and 106 Sm3/h, preferably between 104 and 5xc3x97105 Sm3/h;
it is of the PSA or TSA, preferably TSA, type;
the regeneration temperature ranges between 80xc2x0 C. and 400xc2x0 C., preferably between 100xc2x0 C. and 250xc2x0 C.;
at least a portion of the gas stream stripped of at least some of the impurities is subjected to at least one cryogenic distillation step.
The invention also relates to an adsorbent containing an X-type zeolite having a Si/Al ratio ranging between approximately 1 and approximately 1.25 and containing from 10 to 98% of barium cations and from 2 to 90% of sodium and/or potassium cations, the sum of the barium and potassium and/or sodium cations present in the zeolite representing at least 50% of all of the exchangeable cations.
Preferably, the adsorbent has a Si/Al ratio of about 1.
Advantageously, the adsorbent contains from 70 to 96% of barium cations, the rest of the exchangeable sites being occupied by sodium and/or potassium. cations.